This invention relates to methods for the production of hydrocarbon materials such as oil and natural gas by the recovery of such hydrocarbon materials from subterranean oil bearing reservoirs.
One of the more significant problems attendant to the recovery of hydrocarbon materials from subterranean formations is the concommitant recovery of undesirable fluids such as water. Such recovered water can be brine native to the formation or it can be injection water employed in enhanced oil recovery treatments being applied to the reservoir. Whatever the source, there is an upper limit beyond which water production can no longer be tolerated and its further entry into the producing well bore must be reduced. In addition other fluids such as natural gas are sometimes undesirable particularly when such fluids exceed the upper limits of gas:oil ratio desired for most efficient recovery of the hydrocarbons.
As is well known, many oil reservoirs comprise layers or zones of porous rock which can vary in permeability from less than 10 millidarcies to more than 1000 millidarcies. Regardless of whether the undesired fluid is a natural drive fluid or one coming from an enhanced oil recovery operation, there is a real tendency for the drive fluid to channel along or through the more permeable zones of the formation. Such channeling, often called fingering, generally hinders or prevents the recovery of oil from less permeable zones. For example, the more permeable zones, after oil has been largely displaced therefrom, function as thief zones which permit the drive fluid to channel directly into the production or recovery well.
Among the prior solutions to the problem of undesirable fluid entry into the production well is the placement of a solid plug of a material such as cement within the formation. Unfortunately, such solid plugs often inhibit the use of drive fluids to assist in forcing desired hydrocarbon material from the formation into the producing or recovery well bore. In addition the use of such solid plugs invariably results in the permanent loss of desired fluids. Further, should the undesirable fluids seep by or otherwise bypass such solid plugs, the plug cannot shift position to block such seepage or other changes in flow pattern of the undesired fluid.
In order to overcome the deficiencies associated with the use of solid plugs, it has been a common practice to modify the mobility of the driving fluid by pumping a highly viscous fluid into the oil bearing formation. An illustration of this technique is the incorporation of a partially hydrolyzed polyacrylamide in an aqueous drive fluid such as described in U.S. Pat. No. 3,039,529. While the recovery of hydrocarbon by employing such techniques is measurably enhanced, substantial quantities of polymers must be added to the drive fluid in order to maintain the desired high viscosity. Moreover in cases of highly porous subterranean structures which are proximate to oil bearing formations of relatively low permeability, substantial quantities of the undesirable drive fluid are often recovered.
More recently, as disclosed in U.S. Pat. Nos. 3,780,806 and 3,785,437, it has been a practice to attempt to plug some of the more porous formations by introducing a water soluble polymer into the highly porous structure and crosslinking the polymer in situ to form a water-insoluble gel. Alternatively, as disclosed in U.S. Pat. No. 3,921,733, attempts have been made to pump a partially gelled polymer into a porous formation ahead of the drive fluid. All of these procedures suffer from handling problems characteristic of two component systems comprising a polymer and a crosslinking agent. For example, most of the procedures involve the addition of a dry polymer to an aqueous medium at the well site which gives rise to difficulties in solubilizing the polymer. More importantly, it is difficult to control the gelation characteristics of the polymer as needed to insure effective control of formation permeability.
In view of the deficiencies of prior art methods for controlling permeability in subterranean structures as needed for recovery of hydrocarbon materials, it would be highly desirable to provide an improved oil recovery method which essentially eliminates many of the problems characteristic of the aforementioned in situ gelation techniques.